Avalanche Forecast
Issued: Mar 2nd, 2020 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
Avalanches will be more likely on Tuesday during the morning hours and on the volcanoes due to varied weather inputs including wind, rain, snow, and possibly sunshine. Expect different avalanche problems depending on your elevation with locally lower avalanche danger possible if you’re not on a volcano. Avoid wind-loaded terrain and if you notice the snow is wet, head for slopes less than 35 degrees.
Summary
Snowpack Discussion
February 27, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
In the lead up to last weekend, dry, clear and cold conditions produced surface hoar in many areas as well as faceted snow (check out the National Avalanche Center’s encyclopedia for more details). These weak grain types were not difficult to find and recreationists and professionals across the region began to wonder what role these weak snow surfaces would play when the next storm arrived.
Surface Hoar was not difficult to observe during this period of high pressure. This picture is from 2/21 on an SSE aspect, 5,000’ at Stevens Pass just a day before it may have been buried. Photo: Josh Hirshberg
We didn’t have to wait too long, as a major storm arrived on Sunday 2/23, creating dangerous avalanche conditions for most zones. This system dropped close to 2’ of snow at Mt. Baker while areas to the south along the west slopes of the Cascades saw closer to 1 foot. Areas along the east slopes of the Cascades also saw significant new snow amounts with Blewett Pass and Mission Ridge receiving around 6-8” of fresh snow.
The wind was also a major factor with this storm from about Stevens Pass southward. Multiple stations recorded winds over 80mph and some went over 100mph. This wind was enough to strip surfaces down to old crusts on windward aspects, leaving leeward aspects with wind slabs and touchy cornices. In the case of Mt. Hood, relentless winds this season have stripped the snow down to blue ice from one of our previous atmospheric river events, creating slide for life conditions on certain slopes.
The old snow interface, termed 2/22 for the day it was buried, has us thinking hard about the snowpack moving forward. In the northern part of the region, less wind allowed for more storm than wind slabs, with avalanches that released on both interstorm layers and on the buried 2/22 interface for a day or two following the storm. As you moved further southward or eastward, reactivity on the 2/22 interface was trending downward or non-existent after a natural avalanche cycle that immediately followed the storm. Was the storm potent enough to wipe out the surface hoar and facets? There was a surprising natural avalanche on Stevens Pass during the evening of Monday 2/24, which is suspected to have run on small facets. This avalanche and continued snowpack observations around the region proves that in specific areas, reactive weak snow still existed.
This natural avalanche failed 2ft deep on the 2/22 interface on a SE aspect near 5700ft on Cowboy Mountain. 02/25/20. Photo: Stevens Pass Ski Patrol
On Tuesday, the sun started to shine in some locations, bringing back the threat of wet avalanches and again making us wonder about how warming would affect the 2/22 interface. We never reached the tipping point for widespread wet avalanche activity, but rollerballs and snow shedding off trees and rocks were certainly present in many zones. Drizzle or light snow made an appearance on Wednesday along many west-side zones. Thursday saw a return to warm temperatures and sunny skies all the way from Mt. Hood to Stevens Pass with cooler and cloudier conditions for the North Cascades. All in all, it was a quiet week avalanche-wise and the 2/22 wasn't activated.
We have one more dry mild day before we move into a stormy Saturday. During this stretch, we’ll continue to monitor buried weak snow grains for distribution and reactivity. Check your local forecast zone for the latest.
Problems
Wind Slabs
As you travel up in elevation on Tuesday, you should reach a point where you start to find cold dry snow instead of the wetted snow you may find at lower elevations. When you reach this point, Wind Slabs should be on your mind. Wind Sabs are often found in terrain such as leeward slopes, at the base of cliffs, or the sides of gullies. Look for textured snow surfaces, freshly built cornices, and hollow feeling snow to help identify a potential Wind Slab to steer around. Wind Slabs are typically dense, and may form on top of the lower density snow we received over the weekend. In this scenario, heavier snow would overlay lighter snow, all on top of a firm crust (the 2/28 interface). This is a great setup for slab avalanches, so you should take some time to investigate the bonding between new snow and old snow, as well as the bond to the 2/28 interface. There is a wide range of precipitation forecasted for the zone, so while you’re looking at the bonding, take note of how much new snow has fallen in your location to help you determine a slope scale assessment of the hazard. If you find a poor bond or experience clues of unstable snow such as shooting cracks or whumpfing collapses, head for slopes less than 35 degrees.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
Loose Wet avalanches will be most likely below the forecasted rain line of ~5000’ on Tuesday. Rain falling on the cold dry snow we received over the weekend should be the initial trigger for these avalanches. The main shot of rain should occur overnight on Monday night, possibly lasting into the morning hours which would maintain the hazard. As we move into the afternoon, there is a chance for sun breaks, driving a second round of Loose Wet avalanches. We have less confidence in this second wave of avalanches. If they do occur, it should be in areas where the sun is able to warm and destabilize any cold dry snow surfaces that were not affected by the rain. Once initiated, Loose Wet avalanches often start at a point and can entrain lots of snow as they travel downhill, even triggering slabs as they descend. Rollerballs, pinwheels, and fan-shaped avalanche debris are signs that Loose Wet avalanches are possible. Steer around slopes greater than 35 degrees and be aware of terrain above you if you notice these signs, witness a switch from snow to rain, or feel the snow being warmed by the sun.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Elevations: Treeline, Below Treeline.
Likelihood
Expected Size
Valid until: Mar 3rd, 2020 10:00AM
